The proliferation of portable devices in recent years has highlighted the need for small, efficient rechargeable batteries and battery chargers. As a result, there have been great strides in battery technology. To get the best performance from the batteries that have been developed, designers have dedicated some of their focus to making improvements in battery charger technology. Assuring that battery charger technology keeps pace with advances made in rechargeable battery technology assures the fullest use of a given rechargeable battery's capacity to store and supply electrical power.
Conventional charge systems for rechargeable batteries usually have two phases, a first phase for providing initial charge (when at the beginning of a charge cycle and the battery is completely depleted) and a second phase for providing full rate charge. The second phase is entered into directly when a battery to be charged has at least certain minimum voltage. For one cell Lithium Ion batteries that voltage is about three volts.
It should be appreciated that the magnitude of the charge current can be different at different points in the charge cycle. In some conventional systems, when the battery voltage lies between 0 and 3 volts one current level is generated and above 3 volts another current level is generated. As a result two separate power control devices are needed for pre-charge and full rate charge control current.
FIG. 1 shows a conventional battery charger topology 100. The battery charger shown in FIG. 1 includes a main pass part that comprises p type MOSFET 101, sense resistor 103, and full rate current regulation loop circuit 105, and a pre-charge pass part that comprises charge pre charge control 106, charge pre charge control current reference 107 (e.g., Ipre-charge) and transistors 109 and 111.
Some conventional battery charging systems, such as that shown in FIG. 1, use two charge current pass control devices and two support control circuits. A first pass control device and support circuitry (e.g., 101-105) for the pre-charge charge cycle phase and a second pass control device and support circuitry (e.g., 106-111) for the full rate charge cycle phase. Managing the cooperation between the circuits in order to optimally provide pre-charge and full rate charge current control can pose a significant problem. Many conventional systems do not manage the challenges presented by this circuit structure adequately. As a consequence, some of these systems exhibit low accuracy of pre-charge current and thus do not perform well when they are employed to charge a battery where the charge stored by the battery has been completely depleted.